Wastewater disposal sites at Manteca, California, and Quincy, Washington, were evaluated for their current performance and for the long-term impact of wastewater application. These sites have been operated as slow-infiltration, land-disposal systems for up to 20 years. Current performance was evaluated in terms of water quality, while soil chemical parameters were measured to determine the effects of prolonged wastewater application at the sites. No significant effects on the performance were found to be due to differences in pretreatment. A Difference between the performances of the two sites was attributed mainly to management practices, site history and climatic differences. While leaching of nitrate was observed at both sites, the impact on groundwater quality generally was found to be within the accepted limits ( 10 mg/l of NO3-N). Leaching of phosphorus to a depth of 150 cm was found at both sites but was higher at Manteca. This was thought to be due to problems associated with crop management, land use, and mode and schedule of wastewater application. Total and extractable phosphorus increased in the surface soil layers with time. However, soil nitrogen appeared to decrease, probably because of mineralization. Soil organic matter and cation exchange capacity increased. Some increase in exchangeable Na was noted, but not enough to produce alkaline of saline conditions. A drop in soil pH at Quincy after prolonged application is thought to have been due to removal of carbonates by leaching and by H(+) from nitrification. If these disposal areas were managed as treatment sites, leachate quality should meet proposed Environmental Protection Agency guidelines for drinking water.